For the chemical sensitization of silver halide photographic emulsions, the combinations of labile sulfur and gold compounds are most widely used. The former ones develop on the crystal surfaces silver sulfide centers, from which mainly a gold sulfide or (gold, silver) sulfide arises under the effect of the gold compound. Thus, the sensitivity is increased, the fog is diminished, the reciprocity failure occurring under an illumination of high intensity is improved and the solarization is supressed. A useful labile sulfur compound is inter alia the thiosulfate whereas gold(I) and gold(III) complexes may be employed as gold compounds. [E. Moisar: Phot. Sci. Eng. 25, 45 (1981)].
Considering that gold(III) reacts with gelatin (which, according to most recent investigations, is due to methionine being present in the polypeptide chain), gold(I) complexes are preferred for gold sensitization. The gold(I) isothiocyanate complex formed by the interaction of SCN.sup.- and AuCl.sub.4.sup.- ions, which is the result of an autoreduction in the presence of a high thiocyanate excess, has been used for a long time. Nevertheless, unfavorable properties of the emulsion may also be developed by its use, e.g.: a) the efficiency of sensitization is deteriorated by the chelate formation of gold(I) with gelatin; b) as a result of reducing substances of the gelatin, metallic gold arises which induces fogging; c) due to the thiocyanate being present in excess, a crystal increase occurs also during the chemical ripening whereby the covering ability is deteriorated and the image tone turns into grey in emulsions giving a brown one.
For eliminating these defects other gold compounds have been suggested, such as e.g. dithiosulfate-monoaurate(I) which though better adsorbed on silver halide crystals and having a stronger sensitizing effect, makes the sensitivity distribution strongly heterogeneous and therefore, it gives the possibility of preparing emulsions only with smooth gradation; in addition, depending on the sulfur sensitizer, it may induce fogging, too.
Various water-soluble and water-insoluble gold mercaptides, quaternary gold salts, gold sulfide, gold selenide and the like (U.S. Pat. Nos. 2,597,915, 2,642,361, 3,408,197 and 3,503,749) are also suitable for gold sensitization but their effect does not make the grade of gold thiocyanate or gold thiosulfate.
The supersensitization of sulfur- and gold-sensitized emulsions by palladium(II) salts has been described in the early 50's (U.S. Pat. No. 2,598,079). Nevertheless, the sensitivity was diminished by using [PdCl.sub.4 ].sup.2- together with gold thiosulfate; by using gold thiocyanate alone, the sensitivity was increased, however, the fog was also strengthened [L. De Brabandere and P. Faelens: Phot. Sci. Eng. 25, 63 (1981)]. The sensitivity of the emulsion treated with gold alone was significantly increased by the palladium(II) complexes of some N-alkylated polyamines but its value did not exceed those obtained by sulfur- plus gold-sensitization while the fogging was not changed When the suitable palladium(II) complex was added to a sulfur- plus gold-sensitized emulsion, the maximum sensitivity was not further increased, only the duration of ripening required for reaching the highest sensitivity was shortened whereas the fogging value remained unchanged [J. Hartung et al.: J. Inf. Rec. Mater. 14, 417 (1986)]. In addition to these, a part of the Pd.sup.2+ introduced to the emulsion may also be lost for the sensitization due to the interaction of palladium(II) with gelatin [K. Tanaka: J. Phot. Sci. 21, 134 (1973)] depending on the stability of the palladium(II) complex used.